[PATCH v4 1/1] qemu: add per-vcpu delay stats

[Michal Privoznik]

On 2/19/21 9:08 PM, Aleksei Zakharov wrote:
> This patch adds delay time (steal time inside guest) to libvirt
> domain per-vcpu stats. Delay time is an important performance metric.
> It is a consequence of the overloaded CPU. Knowledge of the delay
> time of a virtual machine helps to understand if it is affected and
> estimate the impact.
> 
> As a result, it is possible to react exactly when needed and
> rebalance the load between hosts. This is used by cloud providers
> to provide quality of service, especially when the CPU is
> oversubscribed.
> 
> It's more convenient to work with this metric in a context of a
> libvirt domain. Any monitoring software may use this information.
> 
> Signed-off-by: Aleksei Zakharov <zaharov at selectel.ru>
> ---
>   docs/manpages/virsh.rst |  4 ++++
>   src/libvirt-domain.c    |  4 ++++
>   src/qemu/qemu_driver.c  | 44 +++++++++++++++++++++++++++++++++++++++--
>   3 files changed, 50 insertions(+), 2 deletions(-)
> 
> diff --git a/docs/manpages/virsh.rst b/docs/manpages/virsh.rst
> index e3afa48f7b..a2b83ddfaa 100644
> --- a/docs/manpages/virsh.rst
> +++ b/docs/manpages/virsh.rst
> @@ -2295,6 +2295,10 @@ When selecting the *--state* group the following fields are returned:
>   * ``vcpu.<num>.halted`` - virtual CPU <num> is halted: yes or
>     no (may indicate the processor is idle or even disabled,
>     depending on the architecture)
> +* ``vcpu.<num>.delay`` - time the vCPU <num> thread was enqueued by the
> +  host scheduler, but was waiting in the queue instead of running.
> +  Exposed to the VM as a steal time.
> +
>   
>   
>   *--interface* returns:
> diff --git a/src/libvirt-domain.c b/src/libvirt-domain.c
> index 4af0166872..e54d11e513 100644
> --- a/src/libvirt-domain.c
> +++ b/src/libvirt-domain.c
> @@ -11693,6 +11693,10 @@ virConnectGetDomainCapabilities(virConnectPtr conn,
>    *     "vcpu.<num>.halted" - virtual CPU <num> is halted, may indicate the
>    *                           processor is idle or even disabled, depending
>    *                           on the architecture)
> + *     "vcpu.<num>.delay" - time the vCPU <num> thread was enqueued by the
> + *                          host scheduler, but was waiting in the queue
> + *                          instead of running. Exposed to the VM as a steal
> + *                          time.
>    *
>    * VIR_DOMAIN_STATS_INTERFACE:
>    *     Return network interface statistics (from domain point of view).
> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
> index b9bbdf8d48..9ec3c8fce7 100644
> --- a/src/qemu/qemu_driver.c
> +++ b/src/qemu/qemu_driver.c
> @@ -1332,6 +1332,34 @@ static char *qemuConnectGetCapabilities(virConnectPtr conn) {
>       return virCapabilitiesFormatXML(caps);
>   }
>   
> +static int
> +qemuGetSchedstatDelay(unsigned long long *cpudelay,
> +                 pid_t pid, pid_t tid)
> +{

I know you took inspiration in existing code, but our standards have 
moved since that code was written, a bit.

> +    g_autofree char *proc = NULL;
> +    unsigned long long oncpu = 0;
> +    g_autofree FILE *schedstat = NULL;
> +
> +    if (tid)
> +        proc = g_strdup_printf("/proc/%d/task/%d/schedstat", (int)pid, (int)tid);
> +    else
> +        proc = g_strdup_printf("/proc/%d/schedstat", (int)pid);
> +
> +    schedstat = fopen(proc, "r");
> +    if (!schedstat) {
> +        virReportError(VIR_ERR_INTERNAL_ERROR,
> +                       _("Unable to open schedstat file at '%s'"),
> +                       proc);

I'd expect this to be an error. Except for the case when the file 
doesn't exist.

> +    }
> +    if (fscanf(schedstat, "%llu %llu", &oncpu, cpudelay) < 2) {
> +        virReportError(VIR_ERR_INTERNAL_ERROR,
> +                       _("Unable to parse schedstat info at '%s'"),
> +                       proc);


And this has to be error always.

> +    }
> +
> +    VIR_FORCE_FCLOSE(schedstat);
> +    return 0;

I'm fixing this function a bit.

> +}

The rest looks okay. I'm pushing it.

Reviewed-by: Michal Privoznik <mprivozn at redhat.com>

Congratulations on your first libvirt contribution!

Michal